I think Pedro, Des and Mortiz have valid points.
And their notes make me think that we have two separate issues:
a) There is no single list of standard values for MT domains, as far as we know.
b) Even if there was, it would likely not be sufficient to cover all cases. (and b is probably the cause for a)
Maybe this could be resolved by a composite value approach:
1) We define a high level list of values based on our 'MT people' input. This would be relatively generic domains, like 'medical', 'sport', 'internet', etc. I hope that just by looking at the lists used in several MT systems we could come up with a list where, if not the labels, at least the concepts would be common.
2) Have a second level value that would be user-defined. It would allow a more fine-grained definition and at the same time allow for customization. The value could use a namespace-like notation to make it unique and possibly point to its definitions.
For example: domain='medical xyz:virology'
This would allow any user agent to have at least one high-level value to fall back to.
This could be implemented as two attributes as well, the notation is a detail, the important point is that we do allow for both: a common set of values, and extension/customization.
Just a thought.
-yves